As a semiconductor material that can be used in a transistor, an oxide semiconductor has been attracting attention. For example, a semiconductor device whose field-effect mobility (in some cases, simply referred to as mobility or μFE) is improved by stacking a plurality of oxide semiconductor layers, among which the oxide semiconductor layer serving as a channel contains indium and gallium where the proportion of indium is higher than the proportion of gallium is disclosed (see Patent Document 1).
A technique for using oxide semiconductor transistors for display devices such as liquid crystal displays and organic electroluminescent (EL) displays has attracted attention. The off-state current of the oxide semiconductor transistors is extremely low. Patent Document 2 and Patent Document 3 each disclose a technique in which such an extremely low off-state current is used to reduce the refresh frequency at the time of displaying still images and the power consumption of the liquid crystal displays or the organic EL displays. Note that such a driving method that reduces the power consumption of display devices is referred to as idling stop (IDS) driving, in this specification.